1. Technical Field
The present invention relates to methods for generating immune responses using adenovirus vectors that allow for multiple vaccination regimens.
2. Description of the Related Art
The most difficult problem with adenovirus vectors has been their inability to sustain long-term transgene expression due largely to the host immune response that eliminates the adenovirus vector and virally transduced cells in immune-competent subjects. Thus, the use of First Generation adenovirus vector vaccines is severely limited by preexisting or induced immunity of vaccines to adenovirus (Ad) (Yang, et al. J Virol 77/799-803 (2003); Casimiro, et al. J Virol 77/6305-6313 (2003)). One group reported that a preponderance of humans have antibody against adenovirus type 5 (Ad5), the most widely used serotype for gene transfer vectors, and that two-thirds of humans studied have lympho-proliferative responses against Ad (Chirmule, et al. Gene Ther 6/1574-1583 (1999)). In another study, an adenovirus vector vaccine carrying an HIV-1 envelope gene was incapable of reimmunizing a primed immune response using non-adjuvanted DNA (Barouch, et al. J. Virol 77/8729-8735 (2003)). Another group reported that non-human primates having pre-existing immunity against Ad5 due to a single immunization with Ad5 were unable to generate transgene-specific antibodies to HIV proteins, as well as altering the overall T cell responses (McCoy, et al. J Virol 81/6594-6604 (2007)).
There are numerous mechanisms by which preexisting immunity interferes with adenovirus vector vaccines but the simplest is the presence of neutralizing antibody followed by cell mediated immune elimination of Ad infected antigen harboring cells. Both of these responses are directed to several Ad proteins. Several approaches have been proposed to overcome the barrier of preexisting anti-vector immunity. Perhaps the most straightforward approach would be to increase the vector vaccine dose. Although there is evidence that increasing vaccine doses can increase induction of desired cell mediated immune (CMI) responses in Ad-immune animals (Barouch, et al. J. Virol 77/8729-8735 (2003)), it often results in unacceptable adverse effects in animals and humans. Consequently, most investigators using First Generation Ad5 vector vaccines use the approach of a heterologous prime-boost regimen, using naked (non-vectored) DNA as the priming vaccination, followed by an Ad5 vector immunization. This protocol also results in a subsequent immune response against Ad5 such that one cannot administer a further re-immunization (boost) with the same (or a different) adenovirus vector vaccine that utilizes the same viral backbone. Therefore, with the current First Generation of Ad5 vectors, using this approach also abrogates any further use of Ad5 vector immunization in the Ad5 immunized vaccinee.
First Generation (E1 deleted) adenovirus vector vaccines express Ad late genes, albeit at a decreased level and over a longer time period than wild-type Ad virus (Nevins, et al. Cell 26/213-220 (1981); Gaynor, et al. Cell 33/683-693 (1983); Yang, et al. J Virol 70/7209-7212 (1996)). When using First Generation adenovirus vectors for immunization, vaccine antigens are presented to the immune system simultaneously with highly immunogenic Ad capsid proteins. The major problem with these adenovirus vectors is that the immune responses generated are less likely to be directed to the desired vaccine epitopes (McMichael, et al. Nat Rev Immunol 2/283-291 (2002)) and more likely to be directed to the adenovirus-derived antigens, i.e., antigenic competition. There is controversy about the mechanism by which First Generation adenovirus vectors are potent immunogens. It has been hypothesized that the composition of the Ad capsid or a toxic effect of viral genes creates generalized inflammation resulting in a nonspecific immune stimulatory effect. The E1 proteins of Ad act to inhibit inflammation following infection (Schaack, et al. PNAS 101/3124-3129 (2004)). Removal of the gene segments for these proteins, which is the case for First Generation adenovirus vectors, results in increased levels of inflammation (Schaack, et al. PNAS 101/3124-3129 (2004); Schaack, et al. Viral Immunol 18/79-88 (2005)). It has been reported that adenovirus vectors efficiently infect antigen-presenting cells (APC) such as dendritic cells and less immunogenic viral vectors do not (Jooss, et al. J Virol 72/4212-4223 (1998)). Antigen presenting cells (APC), such as dendritic cells, are responsible for initiation of CMI responses (Kirk, et al. Hum Gene Ther 11/797-806 (2000)). It has been reported that prevention of gene expression in dendritic cells greatly reduces the intensity of the CMI response (Hartigan-O'Connor, et al. Mol Ther 4/525-533 (2001)).
Thus, it is apparent that there remains a need for a more effective vaccine vector candidate. In particular, there remains a need in the art for Ad vaccine vectors that allow multiple vaccinations and vaccinations in individuals with preexisting immunity to Ad. In addition, there is no homologous vaccine delivery vector that can be employed in a prime reimmunization protocol for vaccination. The present invention provides this and other advantages.